The present invention relates to an improvement of a sync apparatus used for an image multiplex transmission system, such as a teletext system.
In a teletext system, teletext data is transmitted to be superimposed in one predetermined horizontal scanning period of a television broadcast signal. The teletext data superimposed on the television broadcast signal is a so-called teletext signal packet. In the data format of this packet, a clock run-in signal for synchronizing bit units is followed by a framing code for synchronizing byte units and a data portion including various control and teletext data.
Processing for fetching teletext data included in the teletext signal packet from the television broadcast signal on the reception side will be summarized below. First, the teletext signal packet is separated from the television broadcast signal. Next, a sampling pulse synchronous with the clock run-in signal is formed. Finally, the phase of the framing code is detected from data obtained by the sampling pulse, and the data portion is fetched in accordance with the detected phase.
When a series of data-processings starting with separation and sampling of the teletext signal packet, and ending with the storage of the data portion in a buffer memory, is regarded as the fetching of teletext data, the sampling phase and the detected phase of the framing code can be regarded as fetch phases.
A method of determining the fetch phases on the reception side can be largely divided into two methods. In one method, fetch phases are determined for each teletext signal packet (to be referred to as a first method hereinafter).
In the other method, a fetch phase is predetermined in advance. When the actual phase of the teletext signal packet (i.e., the actual phases of the clock run-in signal and the framing code) satisfies a predetermined condition, the fetch phase is re-determined (to be referred to as a second method hereinafter). More specifically, in the second method, when incoincidence between the packet phase and the predetermined fetch phase is successively found a predetermined number of times (M times), a pull-in operation for matching the fetch phase with the packet phase is started. During the pull-in operation, when coincidence between the two phases is successively found a predetermined number of times (N times), the pull-in operation is stopped, and the fetch phase is fixed.
The first method is effective when synchronization of the teletext signal packet has no continuity. Conversely, when synchronization of the teletext signal packet has continuity, the second method can provide a very stable fetch phase. Note that the synchronization continuity of the teletext signal packet means that horizontal and vertical periodicity of the teletext signal packet is stable. In other words, intervals between adjacent superimposed positions of the teletext signal packets are constant.
The synchronization continuity of the teletext signal packets is obtained by driving the teletext signal packets with a sync signal (e.g., a color burst signal). Therefore, since a television signal does not include a color burst signal in black-and-white broadcasting, the teletext signal packets have no synchronization continuity.
In other words, the teletext signal packet is driven by a synchronous portion (e.g., a color burst signal) of a video signal and is superimposed on the video signal. Therefore, when the synchronization of the video signal exhibits discontinuity, the synchronization continuity of the teletext signal packets can no longer be accomplished. In other words, the synchronization continuity of the teletext signal packets is established only when that of the video signal is accomplished.
The necessity of the synchronization continuity will be summarized below.
Reception of the framing code in the teletext system is very important for the entire reception systems in the reproduction of the clock run-in signal, and bit errors in the reception of the framing code are to be as small as possible.
Although the framing code has an error correction function of 1 bit per byte (8 bits), this correction function is insufficient. However, this function can be assisted by flywheel effective reception. In the flywheel effective reception, once a framing code is received by the reception side to synchronize byte units, data is fetched using a framing code of a self-oscillator unless framing errors successively occur. When framing errors successively occur, byte units are re-synchronized using the input framing code (corresponding to the second method).
The function of the framing code can be enhanced by the flywheel effective reception. However, for this purpose, the synchronization continuity of the teletext signal packets (e.g., clock phases, framing code phases between transmission data lines) must be guaranteed. More specifically, after byte units are synchronized by the preceding protective reception, the data is fetched using the framing code of the self-oscillator on the reception side. Therefore, the synchronization continuity of the teletext signal packets is necessary.
To obtain a flywheel effect for bit synchronization also, the synchronization continuity of the teletext signal is indispensable, as in the case of said byte (or packet) synchronization.
A conventional teletext reception apparatus adopts one of the above two methods as a method of determining a fetch phase. As a result, in an apparatus adopting the first method, fetch errors easily occur during color broadcasting having the synchronization continuity when compared with the apparatus adopting the second method. This is because the first method is easily influenced by a noise signal superimposed on the clock run-in signal or the framing code. In the apparatus adopting the second method, since the fetch phase and the packet phase are easily shifted during black-and-white broadcasting with no synchronization continuity, fetch errors of teletext data easily occur when compared with the apparatus adopting the first method.